1. Field of the Invention
The present invention relates to a wire bonding method in which a first bonding point and a second bonding point are connected by a wire, and more particularly to a wire loop formation method in wire bonding.
2. Prior Art
As seen from FIG. 3, in a semiconductor device assembly process, a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (second bonding point) on the lead frame 1 are connected by a wire 3. The connected wire loop having the shape as shown in FIG. 3 is called a trapezoidal loop. Wire loop formation methods of this type is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) Nos. H4-318943 and H7-176558.
The trapezoidal loop shown in FIG. 3 is formed by the process shown in FIG. 4.
In step (a), with a clamper (not shown) which holds the wire 3 opened, the capillary 4 is lowered, so that a ball formed on the tip end of the wire is bonded to the first bonding point A, after which the capillary 4 is raised in substantially vertical direction to point B, and the wire 3 is delivered. Next, in step (b), the capillary 4 is caused to move horizontally to point C in the opposite direction from the second bonding point G. Generally, such movement of the capillary 4 in the opposite direction from the second bonding point G is called a "reverse operation". As a result of the reverse operation, the wire 3 assumes a shape that is inclined from point A to point C, and a kink 3a is formed in a portion of the wire 3. The wire 3 delivered in this process from point A to point C forms the neck height part 31 shown in FIG. 3.
Thereafter, in step (c) in FIG. 4, the capillary 4 is raised in substantially vertical direction to point D, and the wire 3 is delivered. Afterward, in step (d), the capillary 4 is again caused to move horizontally to point E in the opposite direction from the second bonding point G. In other words, a second reverse operation is performed. As a result, the wire 3 assumes a shape that is inclined from point C to point E, and another kink 3b is formed in a portion of the wire 3. This wire 3 delivered from point C to point E forms the trapezoidal length part 32 shown in FIG. 3.
Next, in step (e), the capillary 4 is raised in substantially vertical direction to point F while the wire 3 is delivered. The amount of wire 3 delivered in this case forms the inclined portion 33 shown in FIG. 3. Afterward, the clamper (not shown) is closed. When the clamper is closed, no wire 3 is delivered even of the capillary 4 is subsequently moved. Then, in steps (f) and (g), the capillary 4 is positioned at the second bonding point G by being caused to perform a circular-arc motion or by being lowered after being caused to perform a circular-arc motion, and the wire 3 is bonded to the second bonding point G.
In the trapezoidal loop formation process shown in FIG. 4, the first reverse operation in step (b) is performed with the capillary 4 in a position that is close to the height of the first bonding point A. Accordingly, a comparatively strong kink 3a can easily be formed. However, the second reverse operation in step (d) is performed with the capillary 4 in a high position away from the first bonding point A. Accordingly, the kink 3b is difficult to form and is unstable. As a result, the area of the kink 3b shown in FIG. 3 is unstable, the shape retention strength of the wire loop is weak, and the height of the kink 3b is not aligned with the height of the kink 3a, forming an end-raised or end-lowered loop. Furthermore, if the shape retention strength of the portion in the vicinity of the kink 3b is weak, the bonded wire bends when pressure from the outside is applied to the wire. For example, wire bending tends to occur as a result of external forces such as shocks caused by capillary contact or ultrasonic oscillation during bonding to the second bonding point G, vibration of the wire 3, or mold flow caused by injection of the molding material during molding.